EFFECT OF PRESSURE ON CONDUCTIVITY OF METALS. 87 



out, the wire breaking at the narrow part, and allowing the two pieces 

 to be pulled out separately from either end. 



This method of casting the thermo-couple holes was of course not 

 applicable to the metals with high melting points, and for these copper 

 tubes sweated into place were used exactly as for the central heating 

 element. The inside diameter of these tubes as used for this purpose 

 was 0.016 inch. The junction of the couple was wrapped with fine 

 wire in order to bring its diameter nearly up to the internal diameter 

 of the tube, thereby preventing play in the tube and ensuring better 

 thermal contact at the junction. In this case also the dimensions 

 were such that any error due to flow of heat along the thermo-couple 

 wire from one junction to the other was negligible. The results 

 obtained with specimens made in this way are subject to a correction 

 not easy to calculate for the disturbing effect of the copper tubes on 

 the flow of heat. This correction should not be large because the 



Figure 2. The radial flow specimen attached, with the electrical con- 

 nections, to the insulating plug, ready to screw into the pressure cylinder. 



copper tubes are comparatively small. The centers of the thermo- 

 couple tubes were 0.098 and 0.201 inches respectively from the center 

 of the specimen. 



The electrical connections were got into the cylinder by means of a 

 three-terminal plug of the same design as that used previously in 

 determining the effect of pressure on electrical resistance by the 

 potentiometer method. The only difference is that this plug could be 

 made somewhat larger, because of the larger bore of the pressure 

 cylinder, | against | inch. There is an advantage in this in that the 

 thermo-couple leads can be got a greater distance from the heating 

 element leads, and so there is less danger of heat leaking into the 

 thermo-couple. 



The manner of attaching the specimen to the plug ready for mount- 

 ing in the cylinder is shown in Figure 2. The plug and specimen were 

 attached together, and all electrical connections made before assem- 

 bling so that the whole combination could be screwed into the cylinder 

 as one self-contained unit. Attachments were made to the specimen 



